Laminated timber has been used since the nineteen twenties and have had an ever-increasing importance for structural wood constructions, such as beams and arches for the building industry. This strong evolution has been made possible by modern wood adhesives.
Initially only casein adhesives were used in the manufacturing of laminated timber. Later some urea resin adhesives and cold setting phenol resin adhesives started to be used, but recently the use of resorcinol resin adhesives and resorcinol-phenol resin adhesives has dominated. The reason is the very small aging transformations of these types of adhesives, especially when the wood constructions are exposed to weather. Since these adhesives cure in a neutral or weakly alkaline environment, there is no risk of wood material damage, whereas damage can be caused by cold setting phenol resin adhesives. Furthermore, the curing can be performed at room temperature or at only a slightly raised temperatures so that the risk for building tensions into the wood construction is reduced.
Normally the glueing is conducted in such a manner that a resorcinol-formaldehyde precondensate with deficiency of formaldehyde is mixed with a powdered hardener comprising filler and paraformaldehyde in such an amount that full cross-linking is obtained upon reaction with the precondensate. The powdered paraformaldehyde, which is the active component of the hardener, is continuously solved in the precondensate, whereby formaldehyde is first liberated, which then reacts with the precondensate. A mixture prepared of powdered hardener and precondensate or liquid resin is usually applied to the surfaces to be joined, either by use of a cylinder spreader or a so called bead spreader. A substantial disadvantage in using a mixture of powdered hardener and precondensate or liquid resin is the fairly short pot-life of the mixture. The use of powdered hardener containing filler and paraformaldehyde is furthermore connected with substantial sanitary inconveniences. Such a hardener produces a lot of dust and the finely divided paraformaldehyde present in the air of a work-room where powdered hardener is handled, strongly irrates eyes, nose, throat and lungs.
The relatively short pot-life problems with a mixture of powdered hardener and precondensate or liquid resin can be avoided by separate application of the precondensate or liquid resin and hardener, especially if the hardener is in liquid form. This can for instance be done by a combination of a cylinder spreader first and a bead spreader or with a combination of two bead spreaders arranged one after the other. Upon assembly of the surfaces to be joined and the subsequent press operation a mixing of the components is obtained and the curing reaction can then be completed. The separate application method of the two components also gives considerable handling advantages, since in the separate application method the hardener must be liquid, dust problems do not arise. A liquid hardener is normally composed of formaldehyde solution, the consistency of which, by use of different additives, can be adjusted to give a product which is spreadable from a bead spreader. A problem with this type of product is its strong smell of formaldehyde.
Swedish Pat. No. 7514289-3 discloses the use of a liquid hardener consisting of a stable suspension of finely ground paraformaldehyde in a non-volatile suspending agent such as liquid polyols or mixtures of liquid and solid polyols. By the use of powdered paraformaldehyde the odor problems are essentially avoided and the fact that the powder is dispersed means that the dust problems are avoided. The suspending agents mentioned are compatible with the wood material and with the components of the adhesive and are non-volatile, which is an advantage from the standpoint of stability, hygiene and fire security. However, to avoid the disintegration of paraformaldehyde into formaldehyde, these suspending agents must be as water-free as possible, which can cause troubles due to the hydroscopic nature of these products. It is known to add absorbents for formaldehyde, such as urea, to such mixtures to eliminate the smell of formaldehyde. The reaction occuring between urea and formaldehyde first produces ureaformaldehyde complexes of low molecular weight, which after a certain storage time continue to react with each other to produce resin-like products with higher molecular weight. This means that a liquid hardener for separate bead application with a composition as mentioned above has a tendency to thicken after a certain storage time, whereby problems may arise in spreading a sufficient amount of hardener during the application.
The use of polyols may also eventually cause certain problems in the glued joint. Polyols are compatible with both the resin and the wood material, which is a necessity. However, since the polyols do not participate in the chemical reaction between the resin and the paraformaldehyde, but are only built into the resin structure as an external plasticizer, they can be leached out after a long time outdoors. In such a case a weakened glue joint may be the result. Another possibility is that the polyols may be absorbed by the wood material closest to the glue line. This layer may thereby be softened, which also results in a weakened joint.